Use of Impure Inert Gases in the Controlled Heating and Cooling of Mixed Conducting Metal Oxide Materials

ABSTRACT

Method for processing an article comprising mixed conducting metal oxide material. The method comprises contacting the article with an oxygen-containing gas and either reducing the temperature of the oxygen-containing gas during a cooling period or increasing the temperature of the oxygen-containing gas during a heating period; during the cooling period, reducing the oxygen activity in the oxygen-containing gas during at least a portion of the cooling period and increasing the rate at which the temperature of the oxygen-containing gas is reduced during at least a portion of the cooling period; and during the heating period, increasing the oxygen activity in the oxygen-containing gas during at least a portion of the heating period and decreasing the rate at which the temperature of the oxygen-containing gas is increased during at least a portion of the heating period.

RESEARCH OR DEVELOPMENT

This invention was made with Government support under Cooperative Agreement No. DE-FC26-98FT40343 between Air Products and Chemicals, Inc. and the U.S. Department of Energy. The Government has certain rights to this invention.

BACKGROUND OF THE INVENTION

Ceramic materials containing certain mixed metal oxide compositions possess both oxygen ion conductivity and electronic conductivity at elevated temperatures. These materials, known in the art as mixed conducting metal oxides, may be used in applications including gas separation membranes and membrane oxidation reactors. These ceramic membranes are made of selected mixed metal oxide compositions and have been described as ion transport membranes (ITM). A characteristic property of these materials is that their oxygen stoichiometry is a thermodynamic function of temperature and oxygen partial pressure wherein the equilibrium oxygen stoichiometry decreases with increasing temperature and with decreasing oxygen partial pressure.

It is known that the dimensions of most materials change with changing temperature due to thermal expansion and contraction. In addition to these thermal dimensional changes, mixed conducting metal oxide materials undergo chemical dimensional changes that are a function of the metal oxide oxygen stoichiometry. At isothermal conditions, an article made of mixed conducting metal oxide material will increase in dimensions with decreasing oxygen stoichiometry. At isothermal conditions, the oxygen stoichiometry decreases with decreasing oxygen partial pressure. Since the equilibrium oxygen stoichiometry increases with decreasing temperature, an article made of mixed conducting metal oxides will contract due to both thermal and chemical dimensional changes as the temperature decreases. Conversely, an article made of mixed conducting metal oxides will expand by both thermal and chemical dimensional changes as the temperature increases at a constant oxygen partial pressure. This is described in an article entitled “Chemical Expansivity of Electrochemical Ceramics” by S. B. Adler in J. Am. Ceram. Soc. 84 (9) 2117-19 (2001).

Dimensional changes therefore result from equilibrium oxygen stoichiometry changes in mixed conducting metal oxide materials. Changing the temperature at a constant oxygen partial pressure or changing the oxygen partial pressure at a constant temperature will change the equilibrium oxygen stoichiometry of the mixed conducting metal oxide material. When a mixed conducting metal oxide is used as an ion transport membrane, for example, an oxygen partial pressure difference across the membrane creates a difference in the equilibrium oxygen stoichiometry at each of the two surfaces of the membrane, which in turn creates the thermodynamic driving force for oxygen ions to diffuse through the membrane.

During startup of a gas separation system using mixed conducting metal oxide membranes, the temperature is increased and the oxygen partial pressure on one or both sides of the membrane may change. The equilibrium oxygen stoichiometry of the membrane material will change in response to the changes in temperature and oxygen partial pressure. Oxygen anions will diffuse into or out of the membrane material and the membrane material will approach its equilibrium oxygen stoichiometry value. As the oxygen stoichiometry and temperature changes, the dimension of the membrane will change. The time required for the membrane to reach chemical equilibrium with the oxygen partial pressures on the surfaces of the membrane will depend on the oxygen anion diffusion rate into or out of the membrane. The time required for equilibration to occur is a function of the material composition, the temperature, and the dimension of the membrane modules.

Different membrane compositions will have different oxygen anion diffusivities, and compositions with higher diffusivities will equilibrate with the gas phase faster, all other factors being equal. For a given membrane composition, the oxygen anion diffusivity increases exponentially with temperature. Therefore, equilibration times decrease with increasing temperature. Finally, the equilibration time increases approximately with the square of the characteristic dimension (e.g., length or thickness) of the parts in the membrane modules. Therefore, thinner parts will equilibrate faster than thicker parts, all other factors being equal. As the thickness of a part increases and as the temperature decreases, it becomes increasingly difficult to keep the interior of the part in equilibrium with the gas phase due to sluggish diffusion of oxygen anions into or out of the part.

It is known that temperature gradients in a mixed conducting metal oxide ceramic part can create differential strains due to differential thermal expansion and contraction. Similarly, oxygen stoichiometry gradients in a ceramic part can create differential strains due to differential chemical expansion and contraction. This gradient in oxygen stoichiometry may be sufficiently large to create a correspondingly large differential chemical expansion, and therefore large mechanical stresses, that lead to failure of the part. Therefore, it is desirable to avoid differential chemical expansion or at least to control the differential chemical expansion to below maximum allowable values.

There is a need in applications of mixed conducting metal oxide ceramics for methods to heat or cool ceramic articles at faster rates without producing unacceptable strains in the articles. One method to achieve this is described in U.S. Pat. No. 7,122,072 B2 that discloses a treatment process in which a mixed conducting metal oxide article is contacted with an oxygen-containing gas while the temperature of the gas is increased or reduced. When the temperature of the oxygen-containing gas is increased or reduced, the oxygen activity in the oxygen-containing gas is increased or reduced, respectively, so that the oxygen activity in the gas is substantially in equilibrium with the stochiometric composition of the mixed conducting metal oxide. In this method, a diluent gas may be used to control the oxygen activity during certain periods in the treatment process.

There also is a continuing need in the art for improved methods to reduce the potential for mechanical damage due to chemical dimensional changes during the heating and cooling of articles and systems fabricated from mixed conducting metal oxide materials, both in the manufacturing of parts for such systems and in the operation of gas separation and membrane reactor systems under transient temperature conditions. In particular, there is a need for improvements to these methods in which a diluent gas is used in various process steps during the heating and cooling of mixed conducting metal oxide materials. This need is addressed by the embodiments of the invention disclosed below and defined by the claims that follow. These embodiments are improvements to the method described in U.S. Pat. No. 7,122,072 B2.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the invention includes a method for processing an article comprising a mixed conducting metal oxide material. The method comprises

-   -   (a) contacting the article with an oxygen-containing gas and         either reducing the temperature of the oxygen-containing gas         during a cooling period or increasing the temperature of the         oxygen-containing gas during a heating period;     -   (b) during the cooling period, reducing the oxygen activity in         the oxygen-containing gas during at least a portion of the         cooling period and increasing the rate at which the temperature         of the oxygen-containing gas is reduced during at least a         portion of the cooling period; and     -   (c) during the heating period, increasing the oxygen activity in         the oxygen-containing gas during at least a portion of the         heating period and decreasing the rate at which the temperature         of the oxygen-containing gas is increased during at least a         portion of the heating period.

Another embodiment of the invention relates to a method of operating an ion transport membrane system comprising

-   -   (a) providing at least one membrane module comprising a membrane         made of mixed conducting metal oxide material;     -   (b) contacting the membrane with a heated oxygen-containing gas         comprising dioxygen (O₂) while the temperature of the membrane         is reduced during a cooling period or increased during a heating         period;     -   (c) when the temperature of the membrane is reduced, reducing         the oxygen partial pressure in the oxygen-containing gas and         increasing the rate at which the temperature of the         oxygen-containing gas is reduced during at least a portion of         the cooling period; and     -   (d) when the temperature of the membrane is increased,         increasing the oxygen partial pressure in the oxygen-containing         gas and decreasing the rate at which the temperature of the         oxygen-containing gas is increased during at least a portion of         the heating period.

An alternative embodiment of the invention includes an ion transport membrane system comprising

-   -   (a) at least one membrane module having a membrane comprising         mixed conducting metal oxide material;     -   (b) a gas flow control system adapted to contact the membrane         with a heated oxygen-containing gas and a temperature control         system adapted to either reduce the temperature of the         oxygen-containing gas during a cooling period at any rate of         temperature reduction or increase the temperature of the         oxygen-containing gas during a heating period at any rate of         temperature increase, wherein the gas flow control system and         the temperature control system are adapted to operate in         combination such that;         -   (b1) during the cooling period, reducing the oxygen partial             pressure in the oxygen-containing gas during at least a             portion of the cooling period and increasing the rate at             which the temperature of the oxygen-containing gas is             reduced during at least a portion of the cooling period; and         -   (b2) during the heating period, increasing the oxygen             partial pressure in the oxygen-containing gas during at             least a portion of the heating period and decreasing the             rate at which the temperature of the oxygen-containing gas             is increased during at least a portion of the heating             period.

The mixed conducting metal oxide material may have the general stoichiometric composition (Ln_(1-x)A_(x))_(w)(B_(1-y)B′_(y))O_(3-δ), wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B′ each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr and Ga; wherein 0≦x≦1, 0≦y≦1, and 0.95<w<1.05; and wherein S is a number that renders the compound charge neutral.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a plot of temperature vs equilibrium oxygen partial pressure for a constant stoichiometric composition of a mixed conducting metal oxide ceramic material having the general composition La_(0.4)Sr_(0.6)CoO_(2.84).

FIG. 2 is a schematic flow diagram of a system to control the oxygen partial pressure of the atmosphere in contact with an article comprising mixed conducting metal oxides during heating and cooling of the article.

FIG. 3 is a schematic flow diagram of an alternative system to control the oxygen partial pressure of the atmosphere in contact with an article comprising mixed conducting metal oxides during heating and cooling of the article.

FIG. 4 is a schematic flow diagram of a system to control the oxygen partial pressure of the atmosphere in contact with an ion transport membrane module comprising mixed conducting metal oxides.

FIG. 5 is a schematic flow diagram of a system to control the oxygen partial pressure of the atmosphere in contact with a series of ion transport membrane modules for the separation of oxygen wherein the membrane modules comprise mixed conducting metal oxides.

FIG. 6 is a schematic flow diagram of a system to control the oxygen partial pressure of the atmosphere in contact with a series of ion transport membrane reactor modules wherein the membrane modules comprise mixed conducting metal oxides.

FIG. 7 is a plot of the strain differential between bars of porous and dense La_(0.4)Sr_(0.6)CoO_(3-δ) according to Example 2.

FIG. 8 is a plot of the strain differential between bars of porous and dense La_(0.4)Sr_(0.6)CoO_(3-δ) according to Example 3.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention address the problem of stoichiometrically-related stresses in mixed conducting metal oxide articles and systems during heating and cooling. This problem may be alleviated when a mixed conducting metal oxide material or article is heated or cooled in contact with an oxygen-containing gas by controlling the oxygen activity in the oxygen-containing gas during heating and cooling in a selected temperature range to values that are substantially in chemical equilibrium, or as close as possible to chemical equilibrium, with the oxygen stoichiometry of the mixed conducting metal oxide material while changing the temperature of the material or article in the selected temperature range. Control of the oxygen activity may utilize a diluent gas during certain periods in the heating and cooling steps. This diluent gas typically is an inert gas that is used directly or that may be blended with other process gases.

A high-purity inert gas typically would be preferred for use as the diluent gas. However, a high-purity inert gas may not be available in some situations or locations, and it may be necessary to use an inert gas that contains a small concentration of oxygen as an impurity. The concentration of the oxygen impurity in this inert gas may be in the range of 0.1 ppmv to 5000 ppmv, for example. As described in detail below, the preferred isocompositional profile of a mixed conducting metal oxide article during heating or cooling in the presence of the impure inert gas may not be possible at low temperatures because the required oxygen partial pressures at these temperatures will be less than 10⁻⁵ atm. This will result in the diffusion of oxygen into the article, which will create an oxygen vacancy concentration gradient that causes differential strain through the article, thereby creating stress which can damage the article.

When using low-purity inert gases, it has been found that the differential strain through the article and thus the stress in the article can be reduced significantly by increasing the rate at which the temperature is increased or decreased during at least a portion of the respective heating or cooling steps in a temperature range within which the actual oxygen partial pressure in the isocompositional process is greater than the target oxygen partial pressure. By increasing the rate at which the temperature is increased or decreased (i.e., the temperature ramp rate) at conditions where there is a driving force for oxygen diffusion in the article, less process time is spent at conditions in which oxygen can diffuse into the article. This reduces the amount of oxygen that can diffuse into the article, thereby reducing the chemical expansion stress and potential damage to the article.

The term “activity” as used herein is the thermodynamic function, a, having the usual definition (see, for example, Thermodynamics, G. N. Lewis and M. Randall, revised by K. S. Pitzer and L. Brewer, 2^(nd) Edition, McGraw-Hill, 1961, pp 242-249). The term “total pressure” is defined as the fundamental thermodynamic state variable of pressure as applied to a pure gas or a gas mixture. The term “partial pressure” as applied to a component of a gas mixture is defined as the product of the mole fraction of that component in the gas mixture and the total pressure of the gas mixture. The term “stoichiometric composition” as applied to a chemical compound is the chemical formula that defines the quantititive mass relationships among the elements in the compound.

When the mixed conducting metal oxide material or article is in contact with a gas mixture containing dioxygen or O₂, the oxygen partial pressure may be controlled during heating and cooling in a selected temperature range at values that are substantially in chemical equilibrium, or as close as possible to chemical equilibrium, with the oxygen stoichiometry of the mixed conducting metal oxide material while changing the temperature of the article or system in the selected temperature range. When the oxygen-containing gas is an ideal gas, the oxygen activity is equal to the oxygen partial pressure. The oxygen partial pressure may be controlled in this embodiment by either or both of (1) adjusting the total pressure of the gas mixture and (2) adding to the gas mixture a supplemental gas that has an O₂ concentration different than the O₂ concentration in the gas mixture.

The supplemental gas may be a diluent that contains a lower concentration of O₂ than that in the gas mixture. In the improvements to U.S. Pat. No. 7,122,072 B2 described herein, the diluent gas may be an impure inert gas that contains a low concentration of oxygen in a typical range of 0.1 ppmv to 5000 ppmv. The entire disclosure of U.S. Pat. No. 7,122,072 B2 is incorporated herein by reference.

When the mixed conducting metal oxide material or article is in contact with an oxygen-containing gas that does not contain dioxygen or O₂, the gas-phase oxygen activity may be controlled during heating and cooling in a selected temperature range at values that are substantially in chemical equilibrium, or as close as possible to chemical equilibrium, with the oxygen stoichiometry of the mixed conducting metal oxide material while changing the temperature of the article or system in the selected temperature range. The oxygen activity may be controlled in this embodiment by any of (1) adjusting the total pressure of the gas mixture, (2) adding to the oxygen-containing gas a supplemental gas that has a concentration of an oxygen-containing component different than the concentration of that component in the gas mixture, or (3) replacing at least a portion of the oxygen-containing gas with an inert gas containing a small concentration of oxygen impurity. In the present process embodiment, the diluent is an inert gas selected from the group consisting of nitrogen, argon, helium, water, carbon dioxide, and mixtures thereof.

As used herein, the generic term “oxygen” includes all forms of oxygen comprising the element or moiety having an atomic number of 8. The generic term oxygen therefore includes oxygen ions, gaseous oxygen (dioxygen or O₂), and oxygen that exists in compounds in the gaseous, liquid, or solid state. An oxygen-containing gas is defined as a gas or gas mixture that includes, but is not limited to, one or more components selected from the group consisting of air, nitrogen, O₂, water, carbon monoxide, carbon dioxide, nitric oxide (NO), and nitrous oxide (N₂O). The term “oxygen partial pressure” as used herein means the partial pressure of dioxygen or O₂ in a gas mixture containing O₂ and other gaseous components.

The indefinite articles “a” and “an” as used herein mean one or more when applied to any feature of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated.

In the following description, embodiments of the invention are illustrated by heating and/or cooling methods in which the metal oxide material or article is in contact with an oxygen-containing gas that contains dioxygen or O₂ wherein the oxygen partial pressure is controlled during heating and cooling to be substantially in chemical equilibrium, or as close as possible to chemical equilibrium, with the oxygen stoichiometry of the mixed conducting metal oxide material. The principles described for these embodiments may be applied specifically to embodiments described herein in which O₂ is present as an impurity in an inert gas used during heating and cooling at temperatures below about 1000° C. or below about 600° C. The presence of multiple dimension scales in a single ceramic part may cause differential chemical expansion during isobaric thermal transients in ITM membrane modules and assemblies. For example, U.S. Pat. No. 5,681,373 discloses planar solid-state ITM membrane assemblies comprising multiple layers including a planar porous support with a first surface that is contiguous to a dense separating layer and a second surface that is contiguous to a channeled layer. The diffusion thicknesses or characteristic dimensions of the porous layer, the dense separating layer, and the channel walls in the channeled layer all may be different. This means that the stoichiometric composition of each layer will require different amounts of time to equilibrate with the gas phase oxygen partial pressure during isobaric temperature changes.

The equilibration time of the feature with the largest characteristic dimension, therefore, will determine the heating and cooling rates needed to avoid excessive strain from chemically-induced dimensional changes. The porous layer, which may consist of micron-sized pores defined by micron-sized solid grains of metal oxide material would equilibrate most rapidly with the gas phase. The dense separating layer, which may be as thin as several tens of microns, would equilibrate slower than the ceramic phase in the porous layer. The channel walls, which may be several hundreds of microns in thickness or height, would equilibrate slower than the porous layer and dense separating layer.

If a mixed conducting metal oxide ceramic membrane with these features is cooled at a constant oxygen partial pressure (P_(O2)) at a rate faster than the rate necessary for the channel walls to equilibrate with the gas phase, but slower than the rate for the porous layer to equilibrate with the gas phase, the porous layer will tend to contract faster than the channeled layer. Since the porous layer and the channeled layer are joined to each other, the porous layer will be placed in a tensile stress and the channeled layer in a compressive stress. If the magnitudes of the stresses are large enough, the ceramic part could break. Therefore, methods are needed to allow mixed conducting metal oxide ceramic parts or articles with large characteristic dimensions to be heated and cooled rapidly.

For a given membrane or ceramic article comprising mixed conducting metal oxide material, the differential chemical expansion may be maintained below a maximum allowable value if temperature changes or oxygen partial pressure changes are controlled at a sufficiently low rate such that the part can equilibrate quickly enough to avoid excessive differential chemical expansion. However, as the part becomes thicker or as the temperature decreases, the allowable temperature and oxygen partial pressure rate of change will become slower and slower. These slow temperature or oxygen partial pressure changes may be undesirable, however, because the time required may increase the cost of a process that manufactures or uses articles comprising mixed conductor metal oxide material.

For example, after sintering, ceramic parts must be cooled from the sintering temperature to room temperature, typically in an air atmosphere. To maintain an acceptable differential expansion, the cooling rate may vary from 1° C. per minute at high temperatures to 1° C. per hour at low temperatures. The entire cooling process may require more than one week. The productivity of the sintering furnaces could be improved, and hence the economics manufacturing could be improved, if a higher cooling rate could be used. By eliminating or minimizing solid state oxygen anion transport within the article during thermal transients, differential chemical expansion strains and the mechanical stresses due to those strains may be minimized or eliminated.

As an example, a ceramic part or article having a general stoichiometric composition of La_(0.4)Sr_(0.6)CoO_(3-δ), wherein δ has a value that renders the composition charge neutral, will have an equilibrium stoichiometric composition of La_(0.4)Sr_(0.6)CoO_(2.79) at 900° C. and an oxygen partial pressure (P_(O2)) of 1 atm. A representative equilibrium dimension of this material at these conditions will increase by 757 parts per million (ppm) when the P_(O2) is decreased isothermally from 1 atm to 0.209 atm. A ceramic part that is 1 mm thick at 900° C. and a P₀₂ of 1 atm would therefore expand to a thickness of 1.000757 mm when the P_(O2) is isothermally decreased to 0.209 atm. The equilibrium stoichiometric composition of this material in air at 1 atm total pressure and 100° C. is La_(0.4)Sr_(0.6)CoO_(3.0), while at a P_(O2) of 3.2 atm and 900° C. the equilibrium composition is La_(0.4)Sr_(0.6)CoO_(2.86).

An article comprising a mixed conducting metal oxide material having a certain oxygen stoichiometry may be heated and/or cooled by contacting the article with an oxygen-containing gas while changing the temperature of the article and adjusting the oxygen activity in the oxygen-containing gas to a value that is substantially in chemical equilibrium, or as close as possible to chemical equilibrium, with the stoichiometric composition of the mixed conducting metal oxide material over the temperature range of the heating and/or cooling. Typically, the oxygen-containing gas comprises dioxygen or O₂, and in this case the gas-phase oxygen partial pressure is maintained so that the ceramic article is substantially in chemical equilibrium with the gas phase or is as close as possible to being in chemical equilibrium with the gas phase. In specific embodiments described herein, O₂ is present as an impurity in an inert gas used during the heating and cooling of the article, especially at temperatures below about 1000° C. or below about 600° C.

The terms “article” and “part” are used interchangeably herein and are defined as any element, item, component, or constituent that is fabricated in whole or in part from a mixed conducting metal oxide material. An article or part may consist solely of mixed conducting metal oxide material and may be manufactured by firing green ceramic precursor material in a kiln at sufficiently high temperatures to sinter the precursor and form a ceramic article or part. The article or part may consist of mixed conducting metal oxide material containing a single mixed conducting metal oxide compound or a plurality of mixed conducting metal oxide compounds having different compositions. The article or part may further comprise additional elements or compounds that are not mixed conducting metal oxides including, for example, metals, electronically conductive ceramics, ionically conductive ceramics, and/or non-conducting ceramic materials. In addition, the article or part may be a membrane separator or membrane reactor module that includes mixed conducting metal oxide components as well as piping and other fabrication components made of metal alloys.

If the gas-phase oxygen activity or partial pressure is maintained in chemical equilibrium with the ceramic part or article during heating and/or cooling, oxygen will not diffuse into or out of the article and the oxygen stoichiometry of the article will not change while heating or cooling occurs. This process of heating and cooling at constant composition is defined as isocompositional heating and cooling. By maintaining a constant or nearly constant composition within the ceramic article, the article can be heated or cooled at much higher rates than would be possible if the oxygen stoichiometry of the material in the article changed during heating or cooling. This in turn will improve the productivity of ceramic fabrication while decreasing the mechanical stresses placed on the ceramic articles during manufacture.

These ceramic articles may be components made of mixed conducting metal oxide material wherein the components are parts of membrane modules for oxygen separation and membrane reactor systems. The membrane modules may contain other types of materials in addition to the mixed conducting metal oxide material. The method described above may be used during heating or cooling of membrane modules during startup, shutdown, or any other transient temperature condition. During cooling, the gas phase temperature will be less than the average temperature of the article being cooled, and the gas phase will not be at temperature equilibrium with the article. Conversely, during heating, the gas phase temperature will be greater than the average temperature of the article being heated, and the gas phase also will not be at temperature equilibrium with the article. The objective of controlling the composition of the oxygen-containing gas is to ensure that the overall macroscopic stoichiometric composition of the mixed conducting metal oxide material in the ceramic article remains constant or nearly constant during heating and cooling. During both heating and cooling, the article itself may have an internal temperature gradient and therefore may have localized internal oxygen stoichiometry differences. The oxygen stoichiometry of the article is the overall or macroscopic oxygen stoichiometry of the mixed conducting metal oxide material in the ceramic article.

The term “isocomposition” or “isocompositional” is defined herein as a constant oxygen stoichiometry of a mixed conducting metal oxide material. Isocompositional chemical equilibrium is defined as the chemical equilibrium between an oxygen-containing gas phase and a mixed conducting metal oxide material at a given equilibrium temperature of the gas phase and the mixed conducting metal oxide material. The term “substantially in chemical equilibrium” is defined as the isocompositional chemical equilibrium between the oxygen stoichiometry of a mixed conducting metal oxide material in an article and the gas phase at the gas-phase temperature.

An example of the oxygen partial pressure-temperature equilibrium relationship is shown in FIG. 1 for a mixed conducting metal oxide material having the general stoichiometric composition La_(0.4)Sr_(0.6)CoO_(2.84). This curve is an isocompositional curve of conditions at which the mixed conducting metal oxide is in chemical equilibrium with the gas phase at any temperature on this curve. Other mixed conducting metal oxide compounds will have characteristic isocompositional curves similar to FIG. 1. If the La_(0.4)Sr_(0.6)CoO_(2.84) material of FIG. 1 is initially at 600° C. and a P_(O2) of 0.21 atm, and is cooled to ambient temperature, the material may be maintained at a constant oxygen stoichiometry by changing the P_(O2) as the temperature changes by following the isocompositional curve. For example, as the temperature is decreased from 600° C. to 500° C., the P_(O2) would be decreased from 0.21 atm to 0.016 atm. This may be accomplished, for example, by reducing the total gas pressure in the vessel or diluting air with nitrogen to achieve the desired P_(O2).

As the P_(O2) decreases further, at some point the gas phase oxygen mass transfer kinetics or surface reaction kinetics of the reaction between gas-phase O₂ and the mixed metal oxide may become the rate limiting step for transport of oxygen into or out of the membrane. If this situation occurs, the solid state diffusion of oxygen will be faster than the gas phase O₂ mass transfer kinetics or surface reaction kinetics. As a result, there will be no gradients in the solid state oxygen stoichiometry of the membrane material and hence there will be no mechanical strains due to oxygen stoichiometry gradients. Further reduction of the P_(O2) therefore may be unnecessary as the material is cooled to ambient temperature. The material may be heated from ambient back to 875° C. isocompositionally by following these steps in reverse order.

During certain process steps in which the article is heated or cooled at temperatures below about 1000° C. or about 600° C. in contact with an impure inert gas or diluent containing between 0.1 and 5000 ppmv oxygen, the rate at which the article is heated or cooled is increased or decreased to reduce the time during which the article is in contact with the impure inert gas. The temperature of the article may be changed by increasing or decreasing the temperature of the inert gas and/or increasing or decreasing the temperature of a heating apparatus surrounding the article as described below. The rate at which the temperature of the article, the inert gas, or the heating apparatus is increased may be described as the temperature ramp rate. The inert gas or diluent may comprise one or more components selected from the group consisting of nitrogen, argon, helium, water, carbon dioxide, and mixtures thereof.

The gas phase oxygen partial pressure as a function of temperature is a thermodynamic function of the mixed conducting metal oxide composition and may be determined by conventional means such as thermogravimetric analysis (TGA). The isocompositional curve of FIG. 1 is an example of such an experimentally determined relationship between P_(O2) and temperature. A description of such a technique is given in an article entitled “Nonstoichiometry and Defect Structure of the Perovskite-Type Oxides La_(1-x)Sr_(x)FeO_(3-δ) by J. Mizusaki et. al., J. Solid State Chemistry 58, 257 (1985), which is incorporated herein by reference. Other techniques are equally applicable for this purpose such as, for example, coulometric titration. An example of this technique is disclosed in M. Lankhorst, Ph.D. Thesis, University of Twente, The Netherlands, 1997. In any of these techniques, the equilibrium oxygen partial pressure is measured as a function of temperature for a given oxygen stoichiometry of the mixed conducting metal oxide material to determine the isocompositional P_(O2)-temperature relation of FIG. 1. This isocompositional relation is a fundamental thermodynamic property of the material, and each composition will have a different isocompositional P_(O2)-temperature relationship. For a given mixed conducting metal oxide material composition (i.e., metal cation stoichiometry), each oxygen stoichiometry will have a different isocompositional P_(O2)-temperature relation.

In the manufacturing of articles made of ceramic materials, it is necessary to cool articles to ambient temperature after high-temperature sintering. The principles described above may be used to cool ceramic articles made of mixed conducting metal oxide materials isocompositionally from processing temperatures to ambient temperature. This may be effected in an exemplary apparatus illustrated in FIG. 2. Ceramic articles 201 may be processed in conventional process furnace or kiln 203, which may be an electrically-heated or gas-fired unit of any type known in the art. The furnace temperature may be controlled by a feedback control system comprising temperature sensor 205, controller 207, and feedback signal 209, and such a control system is typically part of the furnace system supplied by the vendor.

The atmosphere in furnace 203 may be controlled by introducing an oxygen-containing gas into the furnace and controlling the composition of the oxygen-containing gas during cooling. The oxygen partial pressure of the oxygen-containing gas may be controlled by introducing a diluent or diluent gas, wherein the diluent or diluent gas used in the improved process described herein may be an inert gas, for example, nitrogen, containing a low concentration of oxygen as an impurity. The oxygen impurity may be present at concentrations typically between 0.1 ppmv and 5000 ppmv, although in some embodiments the oxygen impurity concentration may be outside of this typical range.

The oxygen-containing gas is provided in line 211 at a selected flow rate controlled by flow controller 213 and flows through line 215. The diluent gas is provided in line 217 at a selected flow rate controlled by flow controller 219 and flows through line 221. The oxygen-containing gas and diluent gases may be combined to provide an oxygen-containing gas that flows via line 223 into furnace 203. Vent gas flows from the furnace via line 225, passes through optional gas analyzer 227 and flow control or backpressure regulating valve 229, and may be vented to the atmosphere via line 231 or sent to a vent gas recovery system (not shown) if desired. The gas pressure in furnace 203 may be controlled by pressure control valve 233 and/or regulating valve 229.

In a typical embodiment, the oxygen-containing gas in line 211 is air and the diluent gas in line 217 is nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv. Alternatively, the oxygen-containing gas in line 211 can be oxygen or oxygen-enriched air and the diluent gas may be air, oxygen-depleted air, or nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv. Other combinations of oxygen-containing gas and diluent gases are possible using the oxygen-containing gas and diluent gases listed above. During certain operating periods under certain process conditions, the flow of oxygen-containing gas in line 211 may be terminated and the flow of nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv may be continued via line 217.

The heating or cooling of articles 201 during these periods may be effected by any of (1) heating or cooling (not shown) of the diluent gas in line 217, (2) heating or cooling (not shown) of the oxygen-containing gas in line 211, and (3) changing the temperature of furnace or kiln 203 by controller 207. The use of any of these heating or cooling methods thus will heat or cool the oxygen-containing gas that is in contact with articles 201.

During at least a portion of the cooling period, the process is carried out while increasing the rate at which the temperature of the diluent gas and/or the temperature of the furnace is reduced. During at least a portion of the heating period, the process is carried out while decreasing the rate at which the temperature of the diluent gas and/or furnace is increased. The increase and decrease of the temperature ramp rate (i.e., the rate at which any temperature is increased or reduced) may be effected at temperatures below about 1000° C. or at temperatures below about 600° C.

The flow rate, total pressure, and composition of the oxygen-containing gas supplied to the interior of furnace 203 may be controlled by any of several methods by control system 235. In one embodiment, a furnace atmosphere concentration may be selected and provided as a set point for controller 235, the composition of the furnace atmosphere measured by analyzer 227, and a signal proportional to the O₂ concentration in the furnace effluent relayed via signal line 237 to controller 235. The controller provides set point signals via signal lines 239 and 241 to mass or volumetric flow controllers 213 and 219, respectively, to control the flow rates of the oxygen-containing gas and diluent gas in either or both of lines 215 and 221, respectively. This composition may be constant with time, and alternatively or additionally may change with time in a predetermined time-composition profile or with temperature in a desired temperature-composition profile. Flow controllers 213 and 219 may be individual controllers, each of which has a sufficiently wide range of flow rates. Alternatively, flow controllers 213 and 219 each may comprise multiple control units, each of which covers a portion of the necessary overall range of flow rates.

The total pressure of the atmosphere in furnace 203 may be controlled at a constant value, and alternatively or additionally may be controlled to change with time in a predetermined pressure-time profile or with temperature in a desired pressure-temperature profile. To effect this control, the total pressure is measured by pressure transducer/transmitter 243 and transmitted via signal line 245 to controller 235. The controller regulates the pressure at the predetermined level or pressure-time profile by control signals via signal line 247 and/or signal line 249 to pressure control valve 233 and/or backpressure control valve 229, respectively.

Control system 235 may be programmed to control the oxygen partial pressure or oxygen activity within furnace 203 during heating or cooling according to any predetermined profile. In one embodiment, articles 201 comprising mixed conducting metal oxide material are made of appropriate green ceramic precursors fired in furnace 203 at a typical firing temperature of 1250° C. In order to cool the articles isocompositionally as described earlier (or as close to isocompositionally as possible), the isocompositional P_(O2)-temperature equilibrium relationship for the mixed conducting metal oxide material (similar to FIG. 1) is determined and programmed into controller 235. In this example, the initial atmosphere in the furnace at a typical temperature of 1250° C. is air supplied as the oxygen-containing gas via line 211 and flow controller 213.

In an alternate embodiment, the ceramic articles may be cooled from the firing temperature to a lower temperature at a constant O₂ partial pressure. The values of the lower temperature and the cooling rate are chosen such that the ceramic part will remain in chemical equilibrium, or substantially in chemical equilibrium, with the gas phase over the temperature range from the firing temperature to the lower temperature. Since the oxygen diffusivity is much faster at higher temperatures than at lower temperatures, it is possible to maintain the ceramic parts in chemical equilibrium with the gas phase as the ceramic articles are cooled at a constant oxygen partial pressure from the firing temperature to a given lower temperature. When the parts reach the given lower temperature, controller 235 then would begin to control the total pressure of the furnace or the gas composition of the furnace to a desired temperature-pressure or temperature-oxygen partial pressure profile to maintain the isocompositional oxygen stoichiometry.

The temperature in furnace 203 may be reduced at a predetermined rate (for example, 1° C. per minute) by furnace controller 207, the furnace temperature measured by temperature sensor 251, and a temperature signal transmitted via signal line 253 to controller 235. The temperature then may be compared with the stored isocompositional P_(O2)-temperature equilibrium relationship for the mixed conducting metal oxide material of articles 201 (e.g., FIG. 1). The appropriate P_(O2) is determined and the controller sends the proper set points via signal lines 239 and 241 to flow controllers 213 and 219, respectively. The proper relative flows of air and nitrogen via lines 215 and 221, respectively, are blended and the mixed gas flows via line 223 and control valve 233 to furnace 203, so that the P_(O2) is substantially in equilibrium with the stoichiometric composition of the mixed conducting metal oxide material in articles 201 at the measured temperature. The total pressure typically is maintained at a constant level by controller 235 via pressure control valve 233 and/or backpressure control valve 229. Control by properly decreasing P_(O2) is continued in this manner as the temperature is reduced to about 100° C., below which control of P_(O2) may not be necessary. Alternately, the P_(O2) may become sufficiently low, typically <10⁻⁴ atm, at a higher temperatures that control of the P_(O2) or further reduction in the P_(O2) may not be necessary as the temperature is decreased further.

As an alternative to blending air and nitrogen over the entire cooling temperature range, when the gas pressure in furnace 203 is above atmospheric, the total pressure in the furnace may be reduced by controller 235 via pressure control valve 233 and/or backpressure control valve 229 as the temperature drops during cooling, thereby controlling P_(O2) to maintain an isocompositional state in articles 201. This control of P_(O2) via total pressure control will be effective down to a certain temperature, below which the gas composition may be controlled as described above for the remainder of the cooling.

In an alternative embodiment of the invention, the surface of the mixed conducting metal oxide material in articles or parts 201 may be placed in compression by maintaining the gas phase at an oxygen partial pressure that is less than the equilibrium partial pressure at the gas temperature surrounding the article. The difference between the equilibrium oxygen partial pressure and the oxygen partial pressure set point to be used in this embodiment is first determined by setting the desired degree of surface compressive strain relative to the center of the thickest length scale part, for example 200 parts per million. Then, knowing the amount of strain produced for a given oxygen partial pressure difference at a given temperature for the composition of the ceramic article, the desired temperature-gas composition profile can be calculated and employed during the temperature transient. Intentionally placing the surface of the mixed conducting metal oxide material in compression may be advantageous because ceramics are generally much stronger in compression than in tension. This embodiment also would be advantageous for articles or parts with multiple dimensional scales since the features with the smallest dimensional scales would be placed in compression while the features with the longest dimensional scales would be in tension. However, since features with the largest dimensional scale are generally thicker, the stress levels in the portion in tension would be low.

An alternative embodiment for the control of P_(O2) is illustrated in FIG. 3. In this embodiment, P_(O2) is controlled by combusting a fuel with an oxygen-containing gas such as air to form an oxygen-depleted combustion product which passes to the furnace and provides the atmosphere therein. Ceramic articles 301 are processed in conventional process furnace or kiln 303, which may be an electrically-heated or gas-fired unit of any type known in the art. The furnace temperature may be controlled by a feedback control system comprising temperature sensor 305, controller 307, and feedback signal 309; such a control system typically is part of the furnace system supplied by the vendor.

Oxygen-containing gas (for example, air) is provided in line 311 at a selected flow rate controlled by flow controller 313 and flows through line 315. Fuel gas is provided in line 317 at a selected flow rate controlled by flow controller 319 and flows through line 321. The oxygen-containing gas and the fuel gas are combusted in burner 323 and the oxygen-depleted combustion products flow via line 325 and optional pressure control valve 327 into furnace 303. Flue gas flows from the furnace via line 329, passes through optional gas analyzer 331 and flow control or backpressure regulating valve 333, and may be vented to the atmosphere via line 335 or sent to a flue gas recovery system (not shown) if desired. The gas pressure in furnace 303 may be controlled by control valves 327 and/or 333. The heat generated by burner 323 will supplement the independent furnace heating system, thereby requiring less heat input by this system.

A diluent gas may be introduced via line 337 as required during certain process steps. The diluent gas in line 337 may be an inert gas such as nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv. During certain operating periods under certain process conditions, particularly at temperatures below about 1000° C. or below about 600° C., the flows of oxygen-containing gas in line 311 and fuel in line 317 may be terminated or not initiated, and the flow of nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv may be continued via line 337. Heating or cooling of articles 301 during these periods may be effected by heating or cooling (not shown) of the diluent gas in line 337 and/or changing the temperature of the furnace 303. During at least a portion of the cooling period, the process is carried out while increasing the rate at which the temperature of the diluent gas and/or furnace is reduced. During at least a portion of the heating period, the process is carried out while decreasing the rate at which the temperature of the diluent gas and/or furnace is increased.

The heating or cooling of articles 301 during these periods may be effected by any of (1) heating or cooling (not shown) of the diluent gas in line 337, (2) heating or cooling of the oxygen-containing gas in line 311 by controlling combustion in burner 323, and (3) changing the temperature of furnace or kiln 303 by controller 307. The use of any of these heating or cooling methods thus will heat or cool the oxygen-containing gas that is in contact with articles 301.

The flow rate, total pressure, and composition of the oxygen-containing gas supplied to the interior of furnace 303 may be controlled by any of several methods by control system 339. In one embodiment, a furnace atmosphere concentration is selected and provided as a set point for controller 339, the composition of the furnace atmosphere is measured by analyzer 331, and a signal proportional to the composition of the oxygen-containing gas is relayed via signal line 341 to controller 339. The controller provides set point signals via signal lines 343 and 345 to mass or volumetric flow controllers 313 and 319, respectively, to control the flow rates of the oxygen-containing gas and fuel gas in lines 315 and 321, respectively. This composition may be constant with time and alternatively or additionally may change with time in a predetermined manner. Flow controllers 313 and 319 may be individual controllers, each of which has a sufficiently wide range of flow rates. Alternatively, flow controllers 313 and 319 each may comprise multiple control units, each of which covers a portion of the necessary overall range of flow rates.

The total pressure of the atmosphere in furnace 303 may be controlled at a constant value, and alternatively or additionally may be controlled to change with time in a predetermined pressure-time profile. To effect this control, the total pressure is measured by pressure transducer/transmitter 347 and transmitted via signal line 349 to controller 339. The controller regulates the pressure at the predetermined level or pressure-time profile by control signals via signal line 351 to pressure control valve 327 and/or signal line 353 to backpressure control valve 333, respectively.

Control system 339 may be programmed to control the oxygen partial pressure within furnace 303 during heating or cooling according to any predetermined profile. In one embodiment, articles 301 comprising mixed conducting metal oxide material are fabricated of appropriate green ceramic precursors and fired in furnace 303 at a typical firing temperature of 1250° C. In order to cool the articles isocompositionally as described earlier, the isocompositional P_(O2)-temperature equilibrium relationship for the mixed conducting metal oxide material (similar to FIG. 1) is determined and programmed into controller 339. In this example, the initial atmosphere in the furnace at a typical temperature of 1250° C. is air supplied as the oxygen-containing gas via line 311 and flow controller 313.

The temperature in furnace 303 may be reduced at a predetermined rate (for example, 1° C. per minute) by furnace controller 307 wherein the furnace temperature is measured by temperature sensor 305 and a temperature signal is transmitted to controller 307. The temperature also is measured by temperature sensor 355 and compared with the stored isocompositional P_(O2)-temperature equilibrium relationship (e.g., FIG. 1), the appropriate P_(O2) is determined, and the controller sends the proper set points via signal lines 343 and 345 to flow controllers 313 and 319, respectively. The relative flows of air and fuel via lines 315 and 321 to burner 323 and the total pressure in furnace 303 are controlled so that the P_(O2) is substantially in equilibrium with the stoichiometric composition of the mixed conducting metal oxide material in articles 301 at the measured temperature. The total pressure in the furnace may be maintained at a constant level by controller 339 via pressure control valve 327 and/or backpressure control valve 333. Control by properly decreasing P_(O2) is continued in this manner as the temperature is reduced to about 100° C., below which control of P_(O2) may not be necessary.

As an alternative to or in addition to combusting air and fuel over the entire cooling temperature range, when the gas pressure in furnace 303 is above atmospheric pressure, the total pressure in the furnace may be reduced by controller 339 via pressure control valve 327 and/or backpressure control valve 335 as the temperature drops during cooling, thereby controlling P_(O2) to maintain an isocompositional state in articles 301. This control of P_(O2) via total pressure control will be effective down to a certain temperature, below which the gas composition must be controlled as described above for the remainder of the cooling.

In an alternative embodiment similar to that described above with reference to FIG. 2, the surface of the mixed conducting metal oxide material in articles or parts of membrane module 301 of FIG. 3 may be placed in compression by maintaining the gas phase at an oxygen partial pressure that is less than the equilibrium partial pressure at the gas temperature surrounding the article. Intentionally placing the surface of the mixed conducting metal oxide material in compression may be advantageous because ceramics are generally much stronger in compression than in tension. This embodiment also would be advantageous for articles that have multiple dimensional scales since the features with the smallest dimensional scales would be placed in compression while the features with the longest dimensional scales would be in tension. However, since features with the largest dimensional scale are generally thicker, the stress levels in the portion in tension would be low.

The embodiments of the invention described above are useful in the cooling of articles made of mixed conducting metal oxide ceramic materials from sintering temperatures to ambient temperatures during the manufacturing process. Additional embodiments of this invention may be applied during the heating and/or cooling of process apparatus comprising mixed conducting metal oxide materials. For example, the methods of controlling P_(O2) described above may be applied to the heating and cooling of ceramic membrane gas separation modules and membrane reactor modules during startup, shutdown, and other transient temperature conditions.

This embodiment is illustrated in FIG. 4, which is a schematic flow diagram of a membrane oxygen separation system or a membrane reactor system that utilizes mixed conducting metal oxide membranes. Membrane module 401 is a simplified schematic depiction of a membrane module which comprises oxygen-containing gas feed side 403 and product or permeate side 405 separated by mixed conducting metal oxide membrane 407. Any module configuration or design may be used in this embodiment as long as the membrane comprises a mixed conducting metal oxide material.

Hot oxygen-containing gas in line 409 is introduced into oxygen-containing gas feed side 403 of the module, oxygen permeates membrane 407, and O₂-depleted non-permeate gas exits via line 411. In one embodiment, membrane module 401 may be operated as an oxygen recovery device wherein high purity oxygen collects in product or permeate side 405 of the module and is withdrawn via product line 413. Optional sweep gas may be introduced via line 415. In another embodiment, membrane module 401 may be operated as a membrane reactor in which a reactant gas (for example, a methane-containing gas such as natural gas or a prereformed natural gas/steam/synthesis gas mixture) is introduced via line 415 into membrane module 401. The reactant gas reacts with permeated oxygen in product or reactant side 405 to form a reactant product (for example, synthesis gas) which is withdrawn via line 413.

Membrane module 401 typically operates in the temperature range of 650° C. to 1000° C. and may be heated and maintained at temperature by heating oxygen-containing gas in line 409 in an upstream heating system. In the illustration of FIG. 4, an oxygen-containing gas (for example, air) via line 416 and a fuel (for example, natural gas) via line 417 are combusted in burner 419 to provide the hot oxygen-containing gas in line 409 that is introduced into membrane module 401. Optionally, a diluent (for example, nitrogen or a portion of the O₂-depleted non-permeate from line 411) may be introduced via line 421 and/or line 415.

In an improved embodiment, the diluent gas in line 421 may be an inert gas such as nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv. During certain process periods under certain process conditions, particularly at temperatures below about 1000° C. or below 600° C., the flows of oxygen-containing gas in line 416 and fuel in line 417 may be terminated or not initiated, and the flow of nitrogen that contains oxygen as an impurity in the range of 0.1 to 5000 ppmv may be continued via line 421. Diluent gas also may be introduced via line 415. Heating or cooling (not shown) of membrane 407 during these periods may be effected by heating or cooling (not shown) of the diluent gas in lines 415 and 421. During at least a portion of the cooling period, the process is carried out while increasing the rate at which the temperature of the diluent gas is reduced. During at least a portion of the heating period, the process is carried out while decreasing the rate at which the temperature of the diluent gas is increased.

Alternatively, an oxygen-rich gas (i.e., a gas containing a higher oxygen concentration than the O₂-containing gas in line 409) may be introduced via line 421 and/or line 415. Optionally, indirectly-fired burner or process furnace 423, which is fired by a fuel (for example, natural gas) via line 425 and a oxygen-containing gas (for example, air) via line 427, may be used to further heat the hot oxygen-containing gas from burner 419 without changing the oxygen partial pressure of this gas.

During startup of membrane module 401, the module is heated from ambient temperature to the operating temperature of 650° C. to 1000° C. and during shutdown the module is cooled from this operating temperature to ambient temperature. During the temperature changes accompanying startup and shutdown, it is desirable to control the oxygen partial pressure (P_(O2)) in oxygen-containing gas side 403 of membrane module 401 such that the P_(O2) is substantially in chemical equilibrium, or as close as possible to chemical equilibrium, with the stoichiometric composition of the mixed conducting metal oxide material in membrane 407 over the temperature range of the heating and/or cooling. This may be accomplished by increasing the P_(O2) of the hot oxygen-containing gas in line 409 so that the value of P_(O2) falls on (or as close as possible to) the isocompositional equilibrium line of FIG. 1 as the membrane module is heated, and decreasing the P_(O2) of the hot oxygen-containing gas in line 409 so that the value of P_(O2) falls on (or as close as possible to) the isocompositional equilibrium line of FIG. 1 as the membrane module is cooled.

In another embodiment, the P_(O2) on product side 405 also may be controlled to minimize deviation of that face of membrane 407 from the isocompositional curve of FIG. 1. This may be accomplished by introducing gas of controlled composition to module 401 via line 415. The composition of this gas would be chosen such that the gas-phase composition is substantially in chemical equilibrium with the oxygen stoichiometry of membrane 407. The flow rate of the gas may be chosen to provide sufficient time for the gas-phase composition to change relative to changes in the required equilibrium composition of the gas phase as the temperature changes.

In an alternative embodiment, this technique may be used to control the total pressure on product side 405 as the temperature is changed. For example, at low temperature, the total pressure inside the module may be set to a low value by withdrawing gas through line 413. As the temperature increases, an oxygen-containing gas such as oxygen may be introduced into the module via line 415 to increase the oxygen partial pressure inside the module. During shutdown, the temperature of membrane module 401 is reduced from the operating temperature to ambient temperature. The control of P_(O2) as the module cools may be accomplished in a manner similar to that described above except that during cooling the P_(O2) is decreased to maintain the membrane material as close to the isocompositional line of FIG. 1 as possible.

During steady operation of membrane module 401, a gradient in oxygen partial pressure or oxygen activity typically will exist across membrane 407. Each face of membrane 407 will have a different oxygen stoichiometry. In another embodiment, the P_(O2) or the oxygen activity in oxygen-containing gas side 403 and in the gas in the product side 405 of membrane module 401 may be independently controlled to minimize deviation of each face of membrane 407 from the isocompositional curve of FIG. 1. This may be accomplished by introducing gas of controlled composition to module 401 via line 415 and another gas of controlled composition to module 401 via line 409. The composition of each gas would be chosen so that the gas-phase composition in oxygen-containing gas side 403 is substantially in chemical equilibrium with the oxygen stoichiometry of the membrane 407 on the face of the membrane facing gas side 403 and so that the gas-phase composition in the product side 405 is substantially in chemical equilibrium with the oxygen stoichiometry of the membrane 407 on the face of the membrane facing gas side 405. In this way, the gradient in oxygen stoichiometry may be maintained as the part is cooled or heated. The flow rate of each gas may be chosen to provide sufficient time for the gas-phase composition to change relative to changes in the required equilibrium composition of the gas phase as the temperature changes.

Alternatively, it may be desirable to maintain the surface of the mixed conducting metal oxide material in the membrane module in compression by maintaining the gas phase in oxygen-containing gas feed side 403 of membrane module 401 at an oxygen partial pressure that is less than the equilibrium partial pressure at the gas temperature in oxygen-containing gas feed side 403. Intentionally placing the surface of the mixed conducting metal oxide material in compression may be advantageous because ceramics are generally much stronger in compression than in tension. This embodiment also would be advantageous for membrane module parts with multiple dimensional scales since the features with the smallest dimensional scales would be placed in compression while the features with the longest dimensional scales would be in tension. However, since features with the largest dimensional scale are generally thicker, the stress levels in the portion in tension would be low.

The control of the P_(O2) of hot oxygen-containing gas in line 409 in the above embodiments may be accomplished by appropriate regulation of any or all of the parameters (1) air/fuel ratio in burner 419, (2) firing rate of burner 419, (3) flow rate of optional diluent gas or oxygen-containing gas in line 421, and (4) firing rate of optional indirectly-fired burner or process furnace 423. Additionally or in combination with these parameters, the control of P_(O2) may be effected by controlling the total pressure in oxygen-containing gas feed side 403 of membrane module 401 by appropriate means.

In one possible startup scenario during the heating of membrane module 401 at temperatures above about 100° C., it is desirable to increase P_(O2) as temperature increases as guided by FIG. 1. This may be accomplished, for example, by operating burner 419 at a selected fixed air/fuel ratio and initially at a low firing rate, introducing diluent gas or oxygen-containing gas at an initial rate to achieve the desired P_(O2), and adding supplemental heat as needed by indirect burner 423. As the temperature in membrane module 401 increases, P_(O2) may be increased by changing the flow of diluent gas or oxygen-containing gas via line 421. The firing rate of burner 419 may be increased and the firing rate of burner 423 may be decreased as the final temperature of the membrane module approaches the desired operating temperature. When the membrane module reaches the operating temperature, the flow of diluent gas and operation of burner 423 may be terminated and the system operated with burner 419 operating at the selected air/fuel ratio and supplying all process heating for the system.

Other alternative startup methods are possible and may be devised readily by skilled process engineers and operators using the controllable parameters described above. For example, it may be desirable to fire burner 419 initially at a low air/fuel ratio to provide a lower P_(O2) and then gradually increase the air/fuel ratio to increase P_(O2) as the temperature of membrane module 401 increases. As the membrane module 401 approaches the operating temperature, the firing rate of burner 419 may be decreased and the firing rate of burner 423 may be increased to maintain temperature and a high oxygen partial pressure.

In the embodiment described above with reference to FIG. 4, a single membrane module 401 was used to illustrate embodiments of the invention. The operation of larger commercial-scale membrane oxygen separation systems or membrane reactor systems may require multiple membrane modules operated in series. When membrane modules are operated in series at steady state, a temperature profile and/or a P_(O2) profile may occur in the axial or flow direction over the series of modules; these profiles will be different in gas separation systems and oxidation reactor systems. When a series of membrane modules is started up or shut down, the control of P_(O2) on the feed side of the membrane modules will be more difficult than in a single module because of the axial temperature and/or axial P_(O2) profiles.

The heating or cooling of membrane 407 during in any of the embodiments described above may be effected by any of (1) heating or cooling (not shown) of the diluent gas in line 421, (2) heating or cooling of the oxygen-containing gas in line 416 by controlling combustion in burner 419, (3) heating or cooling of the oxygen-containing gas in line 416 by controlling combustion in burner 423, and (4) heating or cooling of the oxygen-containing gas in line 415 (not shown). The use of any of these heating or cooling methods thus will heat or cool the oxygen-containing gas that is in contact with membrane 407.

Another embodiment of the invention may be utilized to control P₀₂ in series operation of membrane modules wherein different values of P_(O2) are controlled at different axial locations along the series of modules. This is illustrated in FIG. 5, which is a schematic flow diagram of a membrane oxygen separation system that utilizes four mixed conducting metal oxide membrane modules in series. The terms “series” and “series operation” as used herein mean that a process stream leaving any module flows into the next module in the series of modules. In this embodiment, an oxygen-containing gas leaving one module provides the oxygen-containing gas flowing to the next module.

In steady-state operation, preheated oxygen-containing gas (for example, air) may be provided via line 409 from the preheating system of FIG. 4 to membrane module 501, which is shown schematically to include oxygen-containing feed side 501 a, permeate side 501 b, and membrane 501 c. Oxygen permeates membrane 501 c and partially O₂-depleted gas flows via line 503 to membrane module 505, which is shown schematically to include oxygen-containing gas feed side 505 a, permeate side 505 b, and membrane 505 c.

Oxygen permeates membrane 505 c and further O₂-depleted gas flows via line 507 to membrane module 509 which is shown schematically to include oxygen-containing gas feed side 509 a, permeate side 509 b, and membrane 509 c. Oxygen permeates membrane 509 c and further oxygen-depleted gas flows via line 511 to membrane module 513 which is shown schematically to include oxygen-containing gas feed side 513 a, permeate side 513 b, and membrane 513 c. Oxygen permeates membrane 513 c and final O₂-depleted gas is withdrawn via line 515. Permeated oxygen product from the four modules flows via lines 517, 519, 521, and 523 and is withdrawn via manifold 525. Any module configuration or design may be used in this embodiment as long as the membrane comprises a mixed conducting metal oxide material.

During startup, P_(O2) must be controlled in each of the modules 501, 505, 509, and 513 and this may not be possible if P_(O2) is controlled only in the oxygen-containing gas feed in line 409. During heating, a temperature profile may exist in the axial direction (i.e., the flow direction from the inlet of module 501 to the outlet of module 313). Depending on the membrane composition, oxygen permeation may start to be significant at a temperature as low as 400° C. If the oxygen partial pressure is not controlled on both sides of the membranes 501 c, 505 c, 509 c, and 513 c, oxygen may permeate from one side of the membrane to the other. If this occurs, the P_(O2) will be different in the oxygen-containing feed gas side of each module.

In order to control the P_(O2) properly in the oxygen-containing gas feed sides of each module 501, 505, 509, and 513 to minimize deviation from the isocompositional curve of FIG. 1, gas of controlled composition may be introduced to the oxygen containing feed side of each module via lines 409, 527, 529, and 531. The composition of the gas would be chosen such that the gas phase composition is substantially in chemical equilibrium with the oxygen stoichiometry of each module. The flow rate of the gas would be chosen to provide sufficient time for the gas phase composition to equilibrate relative to the changing gas-phase temperature.

In order to control the P_(O2) properly on the permeate sides of the membrane modules 501 through 513 to minimize deviation from the isocompositional curve of FIG. 1, gas of controlled composition may be introduced into each module via lines 517, 519, 521 and 523. The composition of the gas would be chosen such that the gas phase composition is substantially in chemical equilibrium with the oxygen stoichiometry of the membrane. The flow rate of the gas would be chosen to provide sufficient time for the gas phase composition to equilibrate relative to the changing gas-phase temperature.

An alternative embodiment of this technique would control the total pressure on the permeate side of each module as the temperature is changed. For example, while cooling from the operating temperature, the total pressure inside the modules may be decreased by withdrawing gas through line 525 to maintain the oxygen partial pressure in equilibrium with the oxygen stoichiometry of the membrane modules. During a transient when the temperature increases, an oxygen-containing gas such as oxygen may be introduced into each module via lines 517, 519, 521 and 523 to increase the oxygen partial pressure inside each module.

During steady state operation of the system in FIG. 5, axial temperature and/or P_(O2) profiles may exist, that is, the temperatures and/or the stoichiometric compositions of the membranes may differ in the axial or flow direction from module 501 to module 513. When the system is shut down, cooling will begin with these profiles and P_(O2) must be controlled separately in each module during at least part of the cooling period.

In order to control P_(O2) properly in each module 501, 505, 509, and 513 to minimize deviation from the isocompositional curve of FIG. 1, gas of controlled composition may be introduced to the oxygen containing feed side of each module via lines 409, 527, 529, and 531. The composition of the gas would be chosen such that the gas phase composition is substantially in chemical equilibrium with the oxygen stoichiometry of each module. The flow rate of the gas would be chosen to provide sufficient time for the gas phase composition to equilibrate relative to the changing gas-phase temperature.

In order to control P_(O2) properly on the interior of the membrane modules 501 through 513 to minimize deviation from the isocompositional curve of FIG. 1, gas of controlled composition may be introduced into each module via lines 517, 519, 521 and 523. The composition of the gas would be chosen such that the gas phase composition is substantially in chemical equilibrium with the oxygen stoichiometry of the membrane. The flow rate of the gas would be chosen to provide sufficient time for the gas phase composition to equilibrate relative to the changing gas-phase temperature. The composition of the gases in the oxygen-containing feed side and the permeate side of each membrane may be independently controlled to maintain an oxygen stoichiometry gradient across the membrane as the module is cooled or heated. This gradient may be the same oxygen stoichiometry gradient that exists across the membrane during steady state operation at the operating temperature.

An alternative embodiment of this technique would control the total pressure inside each module as the temperature is changed. For example, at steady state operating temperature, the total pressure inside the modules is set to the desired value by withdrawing gas through line 525. As the system is shut down and the temperature decreased, the total pressure would be decreased via line 525 to decrease the oxygen partial pressure inside each module.

In the present improvement to the above method when the membrane modules are heated or cooled at temperatures below about 1000° C. or below about 600° C., an impure inert gas such as nitrogen that contains oxygen as an impurity may be introduced during the heating or cooling steps via any of lines 409, 527, 529, 531 on the feed side and lines 517, 519, 521, and 523 on the permeate side. During these heating or cooling periods, heating or cooling of membranes 501c, 505 c, 509 c, and 513 c may be effected by heating or cooling (not shown) the impure inert gas. During at least a portion of the cooling period, the process is carried out while increasing the rate at which the temperature of the diluent gas is reduced. During at least a portion of the heating period, the process is carried out while decreasing the rate at which the temperature of the diluent gas is increased.

Series membrane modules also may be operated in a membrane reactor system wherein a reactant gas is reacted with permeated oxygen to form selected oxidation products. This is illustrated in FIG. 6 in which four membrane reactor modules 601, 603, 605, and 607 are operated in series. The modules comprise, respectively, reactant sides 601 a, 603 a, 605 a, and 607 a, membranes 601 b, 603 b, 605 b, and 607 b, and oxygen-containing gas feed sides 601 c, 603 c, 605 c, and 607 c. Any module configuration or design may be used in this embodiment as long as the membrane comprises a mixed conducting metal oxide material. A heated oxygen-containing gas (for example, air) is introduced to the system via inlet line 409, manifold 608, and individual feed lines 609, 611, 613, and 615 into oxygen-containing gas feed sides 601 c, 603 c, 605 c, and 607 c, respectively. This heated oxygen-containing gas may be provided by the preheating system described earlier with reference to FIG. 4. O₂-depleted non-permeate gas is withdrawn from each module via manifold 616.

Preheated reactant gas (for example, a methane-containing gas such as natural gas or a prereformed natural gas/steam/synthesis gas mixture) is introduced via line 617 and flows in series through the reactant sides of modules 601, 603, 605, and 607 while reacting therein with oxygen that has permeated through membranes 601 b, 603 b, 605 b, and 607 b, respectively. Reaction product gas (for example, synthesis gas comprising hydrogen and carbon monoxide) is withdrawn from the system via line 619. During steady-state operating conditions, the temperature and/or the stoichiometric composition of the membranes may differ in the axial or flow direction from module 601 to module 607. When the system is shut down, cooling will begin with these profiles and P_(O2) must be controlled separately in each module during at least part of the cooling period

The terms “series” and “series operation” as used herein means that a process stream leaving any module flows into the next module in the series of modules. In this embodiment, the reactant gas leaving one module provides the reactant gas flowing to the next module.

The heated oxygen-containing gas, typically air, enters via line 409 and flows to the modules as described above. The oxygen partial pressure in the oxygen-containing gas to each module may be controlled during heating or cooling of modules 601, 603, 605, and 607 by introducing additional gas via lines 621, 623, 625, and 627, respectively. When the modules are cooled, for example during system shutdown, the oxygen partial pressure in oxygen-containing feed gas sides 601 c, 603 c, 605 c, and 670 c may be reduced by introducing a diluent gas via lines 621, 623, 625, and 627, respectively. When the modules are heated, for example during system startup, the oxygen partial pressure in oxygen-containing feed gas sides 601 c, 603 c, 605 c, and 670 c may be increased by introducing an O₂-enriched gas via lines 621, 623, 625, and 627, respectively.

Alternatively, during system startup, the oxygen partial pressure in oxygen-containing feed gas sides 601 c, 603 c, 605 c, and 607 c may be controlled by blending an oxygen-containing gas such as air from line 409 with a diluent gas via lines 621, 623, 625, and 627, respectively. The total pressure of the oxygen-containing gas in oxygen-containing feed gas sides 601 c, 603 c, 605 c, and 670 c may be controlled by known means as necessary during heating, steady state operation, and cooling of the module system.

In the present improvement to the above method at temperatures when the membrane modules are heated or cooled at temperatures below about 1000° C. or below about 600° C., an impure inert gas such as nitrogen that contains oxygen as an impurity may be introduced during the heating or cooling steps via any of lines 617, 621, 623, 625, and 627. During these heating or cooling steps, heating or cooling of membranes 601 b, 603 b, 605 b, and 607 b may be effected by heating or cooling (not shown) the impure inert gas. During at least a portion of the cooling period, the process is carried out while increasing the rate at which the temperature of the diluent gas is reduced. During at least a portion of the heating period, the process is carried out while decreasing the rate at which the temperature of the diluent gas is increased.

The following Examples illustrate embodiments of the present invention but do not limit the invention to any of the specific details described therein.

EXAMPLE 1 Preparation of Mixed Conducting Metal Oxide Test Specimens

Dense bars were prepared by uniaxially pressing powder of La_(0.4)Sr_(0.6)CoO_(3-δ), sintering at 1250° C. for 4 hours, and cooling to room temperature. A porous bar of the same composition was made by uniaxially pressing powder of La_(0.4)Sr_(0.6)CoO_(3-δ), sintering at 1000° C. for 1 hour, and cooling to room temperature. The bars were machined to the dimensions given in Table 1 below.

TABLE 1 Dimensions of Samples Porous Bar Dense Bar Width (mm) 4.00 2.14 Thickness (mm) 1.84 1.38 Length (mm) 25.22 26.12

Example 2 Effect of Inert Gas Purity on the Dimensional Changes of Mixed Conducting Metal Oxide Samples During Isocompositional Cooling

A 2-stage dilatometer (Linsies) was used to measure simultaneously the dimensional changes in the same atmosphere of a porous bar and a dense bar made in Example 1. The bars were initially heated in air to 900° C. at 0.5° C./min, held at 900° C. for ½ hr, and then cooled in air at 0.5° C. to 600° C. At that point the bars were isocompositionally cooled using nitrogen containing either 146 ppmv of O₂ or 52 ppmv of O₂. The onset of isocompositional cooling was 600° C., which means that P_(O2) in the gas was controlled by blending air and inert gas to maintain the oxygen vacancy fraction at the same value that is in equilibrium with 1 atm air at 600° C. A constant temperature ramp rate of 0.5° C./min was used when cooling from 600° C. to ambient conditions.

The rigid band parameters given in Table 2 were used to calculate the P_(O2) required during isocompositional cooling using the equations described below. The variation of oxygen partial pressure as a function of temperature, to maintain a constant equilibrium vacancy concentration, x_(v)(T*,0.21), is given in the following equation:

${\ln \left( P_{O_{2}} \right)} = \frac{\begin{matrix} {E_{O_{x}} - {\frac{4}{g\left( ɛ_{f} \right)}\left( {{6{x_{v}\left( {T^{\prime},0.21} \right)}} - x} \right)} -} \\ {{TS}_{O_{x}} - {2{RT}\; {\ln \left( \frac{x_{v}\left( {T^{\prime},0.21} \right)}{1 - {x_{v}\left( {T^{\prime},0.21} \right)}} \right)}} - {\mu_{O_{2}}^{0}(T)}} \end{matrix}}{RT}$

where x_(v)(T*,0.21) represents the isocompositional oxygen vacancy concentration to be held constant at isocompositional temperature, T*, and Po₂ of 0.21 atm. In this case T* is 600° C. The chemical potential of oxygen, μ⁰ _(O2), is given by:

${\mu_{O_{2}}^{0}(T)} = {{RT}\left\lbrack {{- 1.225} - \frac{1045}{T} - {3.5{\ln (T)}} + {1.013\ln \left\{ {1 - {\exp \left( \frac{- 2242}{T} \right)}} \right\}}} \right\rbrack}$

where T is the absolute temperature and R is the ideal gas constant. The parameters Eo_(x), 4/g(ε_(f)), x, and So_(x) are given in Table 2 below.

TABLE 2 La_(0.4)Sr_(0.6)CoO_(3-δ) Rigid band model parameters used in isocompositional cooling experiments. Value Parameter (Po₂ < 1 atm) Units Eo_(x) −255790 J/mol 1/g(ε_(f)) 57694.6 J/mol x 0.6 So_(x) 96.73 J/(mol K)

During isocompositional cooling, the target O₂ concentration goes below 150 ppmv at 395° C. Using an inert gas with a O₂ concentration of 150 ppmv therefore results in an atmosphere that contains more oxygen than desired when the temperature is <395° C. This results in differential strain between the porous and dense bars because the porous bar is able to take up oxygen faster than the dense bar, thereby resulting in additional chemical contraction of the porous bar at temperatures <400° C. Using a faster cooling rate will result in less oxygen uptake by the La_(0.4)Sr_(0.6)CoO_(0.3-δ) and hence less differential strain between the porous and dense bars. In an oxygen separation module, this should result in less tensile stress on the surface of the membranes in the module that is isocompositionally cooled in an atmosphere with higher levels of oxygen in the inert gas.

FIG. 7 gives a comparison of isocompositional cooling using 146 ppmv O₂ in N₂, isocompositional cooling using 52 ppmv O₂ in N₂, and cooling in air, all at a constant cooling rate of 0.5° C./min. This shows that the purer the inert gas, the lower the differential strain that can be achieved using isocompositional cooling. At a constant cooling rate of 0.5° C./min, the differential strain between the porous bar and the dense bar was only 300 ppmv when the inert gas contains 52 ppmv of O₂ but increases to 600 ppmv when the inert gas contains 146 ppmv of O₂.

EXAMPLE 3 Effect of Increasing Temperature Ramp Rate

A 2-stage dilatometer (Linsies) was used to measure simultaneously the dimensional changes of the two bars of Example 3 in the same atmosphere. The bars were initially heated in air to 900° C. at 0.5° C. /min, held at 900° C. for ½ hr, and then cooled in air at 0.5° C. to 600° C. At that point the bars were isocompositionally cooled using an inert gas with 146 ppmv of O₂. The onset of isocompositional cooling was 600° C., and P_(O2) in the gas was controlled during cooling by blending air and inert gas to maintain the oxygen vacancy fraction at the same value that is in equilibrium with 1 atm air at 600° C. The rigid band parameters given in Table 2 were used as described in Example 2 to calculate the P_(O2) required during isocompositional cooling. A temperature ramp rate of 0.5° C. /min was used to 400° C., at which point the ramp rate was increased to 2° C./min.

During isocompositional cooling, the target gas phase O₂ concentration fell below 150 ppmv at 395° C. Using an inert gas with a O₂ concentration of 150 ppmv therefore results in an atmosphere with more oxygen than desired when the temperature is <395° C. As shown in Example 2, this results in differential strain between a porous and dense La_(0.4)Sr_(0.6)CoO_(3-δ) bar as the porous bar is able to take up oxygen faster than the dense bar resulting in additional chemical contraction of the porous bar at temperatures <400° C.

FIG. 8 compares the results when a cooling rate of 2° C./min was used during isocompositional cooling at T<400° C. versus a cooling rate of 0.5° C./min that was used during isocompositional cooling at T<400° C. when the inert gas contained 146 ppmv O₂. Using the faster cooling rate decreased the differential strain from 612 ppmv to 445 ppmv. This indicates that using the process of the present invention decreased the differential chemical expansion between a porous bar and a dense bar, which resulted in less stress in the dense bar. As an added benefit, increasing the temperature ramp rate made the cooling process faster and decreased the process cooling time. 

1. A method for processing an article comprising a mixed conducting metal oxide material, which method comprises (a) contacting the article with an oxygen-containing gas and either reducing the temperature of the oxygen-containing gas during a cooling period or increasing the temperature of the oxygen-containing gas during a heating period; (b) during the cooling period, reducing the oxygen activity in the oxygen-containing gas during at least a portion of the cooling period and increasing the rate at which the temperature of the oxygen-containing gas is reduced during at least a portion of the cooling period; and (c) during the heating period, increasing the oxygen activity in the oxygen-containing gas during at least a portion of the heating period and decreasing the rate at which the temperature of the oxygen-containing gas is increased during at least a portion of the heating period.
 2. The method of claim 1 wherein the article has a stoichiometric composition, the article is cooled, the oxygen activity in the oxygen-containing gas is reduced while cooling the oxygen-containing gas so that the oxygen activity is substantially in chemical equilibrium with the stoichiometric composition of the mixed conducting metal oxide material during at least a portion of the cooling period, and the rate at which the temperature of the oxygen-containing gas is reduced is increased during at least a portion of the cooling period.
 3. The method of claim 1 wherein the article has a stoichiometric composition, the article is heated, and the oxygen activity in the oxygen-containing gas is increased during heating so that the oxygen activity is substantially in chemical equilibrium with the stoichiometric composition of the mixed conducting metal oxide material during at least a portion of the heating period, and the rate at which the temperature of the oxygen-containing gas is increased is reduced during at least a portion of the cooling period.
 4. The method of claim 1 wherein the oxygen-containing gas comprises a diluent and wherein the oxygen activity in the oxygen-containing gas is changed by changing concentration of the diluent therein.
 5. The method of claim 1 wherein the oxygen activity in the oxygen-containing gas is changed by changing the total pressure of the oxygen-containing gas.
 6. The method of claim 1 wherein the oxygen-containing gas comprises one or more components selected from the group consisting of oxygen, nitrogen, nitric oxide, nitrous oxide, carbon monoxide, carbon dioxide, and water.
 7. The method of claim 4 wherein the diluent comprises one or more components selected from the group consisting of nitrogen, argon, helium, water, carbon dioxide, and mixtures thereof and wherein the diluent contains between 0.1 ppmv and 5000 ppmv of oxygen.
 8. The method of claim 4 wherein the oxygen-containing gas comprises dioxygen (O₂) and nitrogen.
 9. A method of operating an ion transport membrane system comprising (a) providing at least one membrane module comprising a membrane made of mixed conducting metal oxide material; (b) contacting the membrane with a heated oxygen-containing gas comprising dioxygen (O₂) while the temperature of the membrane is reduced during a cooling period or increased during a heating period; (c) when the temperature of the membrane is reduced, reducing the oxygen partial pressure in the oxygen-containing gas and increasing the rate at which the temperature of the oxygen-containing gas is reduced during at least a portion of the cooling period; and (d) when the temperature of the membrane is increased, increasing the oxygen partial pressure in the oxygen-containing gas and decreasing the rate at which the temperature of the oxygen-containing gas is increased during at least a portion of the heating period.
 10. The method of claim 9 wherein the membrane has a stoichiometric composition, the membrane is cooled, the oxygen partial pressure in the oxygen-containing gas is reduced while cooling the membrane so that the oxygen partial pressure is substantially in chemical equilibrium with the stoichiometric composition of the membrane during at least a portion of the cooling period, and the rate at which the temperature of the oxygen-containing gas is reduced is increased during at least a portion of the cooling period.
 11. The method of claim 9 wherein the membrane has a stoichiometric composition, the membrane is heated, the oxygen partial pressure in the oxygen-containing gas is increased while heating the membrane so that the oxygen partial pressure is substantially in chemical equilibrium with the stoichiometric composition of the membrane during at least a portion of the heating period, and the rate at which the temperature of the oxygen-containing gas is increased is decreased during at least a portion of the heating period.
 12. The method of claim 9 wherein the oxygen partial pressure in the oxygen-containing gas in contact with the membrane is changed by mixing a diluent gas with the oxygen-containing gas.
 13. The method of claim 9 wherein the oxygen partial pressure in the oxygen-containing gas in contact with the membrane is changed by changing the total pressure of the oxygen-containing gas.
 14. The method of claim 9 wherein the heated oxygen-containing gas is provided by the direct combustion of an oxidant gas comprising dioxygen (O₂) with a fuel.
 15. An ion transport membrane system comprising (a) at least one membrane module having a membrane comprising mixed conducting metal oxide material; (b) a gas flow control system adapted to contact the membrane with a heated oxygen-containing gas and a temperature control system adapted to either reduce the temperature of the oxygen-containing gas during a cooling period at any rate of temperature reduction or increase the temperature of the oxygen-containing gas during a heating period at any rate of temperature increase, wherein the gas flow control system and the temperature control system are adapted to operate in combination such that; (b1) during the cooling period, reducing the oxygen partial pressure in the oxygen-containing gas during at least a portion of the cooling period and increasing the rate at which the temperature of the oxygen-containing gas is reduced during at least a portion of the cooling period; and (b2) during the heating period, increasing the oxygen partial pressure in the oxygen-containing gas during at least a portion of the heating period and decreasing the rate at which the temperature of the oxygen-containing gas is increased during at least a portion of the heating period.
 16. The system of claim 15 wherein the mixed conducting metal oxide material has the general stoichiometric composition (Ln_(1-x)A_(x))_(w)(B_(1-y)B′_(y))O₃₋₆, wherein Ln represents one or more elements selected from La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from Mg, Ca, Sr and Ba; wherein B and B′ each represent one or more elements selected from Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Cr, Al, Zr and Ga; wherein 0≦x≦1, 0≦y≦1, and 0.95<w<1.05; and wherein δ is a number that renders the compound charge neutral.
 17. The system of claim 16 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La_(x)Ca_(1-x))_(w)FeO_(3-δ) wherein 1.0>x>0.5, 1.1≧w≧1.0, and δ is a number which renders the composition charge neutral.
 18. The system of claim 16 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La_(x)Sr_(1-x))_(w)CoO_(3-δ) wherein 1.0>x>0.1, 1.05≧w>0.95, and δ is a number which renders the composition charge neutral.
 19. The system of claim 18 wherein the mixed conducting metal oxide material has the general stoichiometric composition (La_(0.4)Sr_(0.6))_(w)CoO_(3-δ) wherein 1.05≧w>0.95 and δ is a number which renders the composition charge neutral.
 20. The system of claim 15 which further comprises a combustor for combusting an oxidant gas comprising dioxygen (O₂) with a fuel to generate a combustion product that provides the heated oxygen-containing gas. 